The growth and health of plants relies on a variety of nutrients. Three nutrients in particular are common components of fertilizer: nitrogen (N), phosphorus (P), and potassium (K). Fertilizers may additionally contain other active materials including macronutrients, such as magnesium (Mg), calcium (Ca), sulfur (S), micronutrients, such as boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn), and nickel (Ni), pesticides, herbicides, etc.
A problem with many fertilizer compositions is the lack of sustained and simultaneous availability to plants of the component nutrient sources in the fertilizer. Typically, water-soluble components rapidly permeate the soil and may be lost via leaching, run-off or chemical binding with soil minerals. Water-insoluble components may be released over longer time scales.
Availability and retention of the nutrients and other active materials within the fertilizer will also be influenced by other factors. For instance, the pH level of soil and water hardness affect the bioavailability of phosphorus because phosphorus forms insoluble precipitates when sequestered by calcium within alkaline soils and aluminum or iron within acidic soil. Irrigation with hard water is another factor that can limit the effectiveness of fertilizers to deliver phosphorus since hard water has high calcium content. A fertilizer that releases phosphorus at a lower rate (a slow-release fertilizer) can provide plants with a better opportunity to uptake this nutrient. Improved nutrient uptake increases nutrient use efficiency and decreases the amount of fertilizer required for optimum plant growth over a growing season. Furthermore, by decreasing the amount of fertilizer used and increasing the efficiency of nutrient uptake, less fertilizer may be used, and this may reduce the impact on the environment.
Various controlled or slow release fertilizers are described in the literature. These include: Agrotain™, Environmentally Smart Nitrogen (ESN™), Osmocote™, Nutricote™, and Polyon™.
Another issue of importance is the cost of fertilizers. It would be desirable to produce a fertilizer from low-cost raw materials. Struvite is a material that can be obtained as a by-product of waste water treatment processes. Harvesting struvite from wastewater is described for example in U.S. Pat. Nos. 7,622,047 and 8,444,861. Struvite has the formula MgNH4PO4.6H2O and is also known as magnesium-ammonium-phosphate. Due to its low solubility in water, the slow-release nature of struvite can provide a sustained source of phosphorus over a growing season. The slow release of phosphorus also minimizes the sequestration of phosphorus by soil-borne cations as the phosphorus will only be made available by dissolution when there are plant roots present to take up the phosphorus; thus, by reducing the time phosphorus is present in the soil, the amount of sequestration by soil-borne cations is reduced. Furthermore, the presence of magnesium within struvite alleviates concerns over ineffectual phosphorus uptake by plants grown in alkaline soils or irrigated with hard water. Thus, struvite is an abundant, renewable source of water-insoluble phosphorus that may be used in the production of fertilizers.
Patent publications relating to fertilizers include:                US20060230798 discloses a fertilizing material comprising a phosphate fertilizer and a phosphate binder of a bauxite refinery residue known as red mud,        US20130104612 describes a phosphate fertilizer compound prepared from an insoluble source of phosphorus and preferably phosphate rocks in the presence of complexing compounds extracted from organic matter, and in particular the presence of humic substances to yield organo-calcium-phosphate complexes.        U.S. Pat. No. 8,262,765 discloses methods of producing a controlled release fertilizer that include intentional formation of magnesium ammonium phosphate within amino acid fermentation byproduct liquors.        US20100326151 discloses an agglomerated fertilizer product comprising of nutrients and inert solids derived from a source material such as manure or effluent.        US20110314882 discloses A fertiliser composition in the form of pellets or granules comprising an organic fertiliser in admixture with a water absorbent polymer.        WO2012020427 discloses a fertilizer comprising a water-insoluble, dilute acid-soluble polyphosphate composition.        WO2012053907 discloses a mixed release rate fertilizer composition comprising an acidulated phosphate together with a source of both slow release cobalt and quick release cobalt.        
There is a need for fertilizers that can supply plants with nutrients, especially phosphorus, on both an immediate and ongoing basis.